AIR FLOW AND PARTICLE TRANSPORT IN A TRIBOELECTRIC COAL/ASH CLEANING SYSTEM-COUNTER FLOWING STRAIGHT DUCT DESIGN

Abstract

ABSTRACT The process of triboelectric coal/ash cleaning is outlined and a new design for scale-up to industrial systems is described. A straight rectangular duct forms the mixing chamber and the main body of this triboelectric separator. To improve mixing and to generate a uniform concentration in the main electrostatic separator duct, the mixture enters the mixing box in the form of two opposing jets. The stress transport model of the FLUENT code is used for simulating the mean gas flow and to guide the overall design of the electrostatic coal/ash separator. The three-dimensional computational grid covers the mixing chamber with the opposing inlets and the upper part of the main separator duct. The gas flow field conditions including the directional intensities of turbulence are evaluated. The instantaneous fluctuating velocity field was simulated by a Gaussian filtered white noise model. A recently developed computational model for Lagrangian particle tracking is used to study the transport of particles from the inlets to the main body of the separator duct. The model accounts for the drag and lift forces acting on the particle, in addition to the Brownian motion and gravitational sedimentation effects. The particles are also dispersed by the action of the high level of turbulence that is generated in the mixing chamber. The computational results show that the new design generates roughly uniform flow conditions in the triboelectric separator duct. Furthermore, the larger 100 µm particles will have a more uniform distribution when compared with the smaller (40 µm) particles. The effect of the presence of an electric field perpendicular to the flow direction is also studied. It is shown that the charged particles are significantly deflected by the action of the electrostatic forces. As a result, the system could separate coal and ash particles that carry different charges. The simplicity and effectiveness of the straight duct separator makes it a prime candidate for scale-up for use in industrial applications for online coal beneficiation and separation of minerals from ash in combustion product.